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Trouble Shooter

As vehicles have evolved, some diagnostic problems have faded into memory. But some hardy perennials, like parasitic battery drain, are still with us. Sorting out the cause may be harder now than in the past.

Circling the Drain

Last month in this column we discussed several diagnostic methods that may be used to find the source of an abnormal key-off battery drain. These kinds of abnormal loads go by different names. Some call them parasitic loads, while others call them key-off battery draws or drains. If you like, you can roll all of them together and call them parasitic battery drains. Regardless of what you call them, we can all agree that finding the source of such an abnormal key-off battery draw can be a major pain.

At the end of the column, I described a diagnostic method for finding the cause of a key-off battery drain that was new to me. This method involves using a DVOM to measure the voltage drop across individual fuses. As we know, there will be no measurable voltage drop in a circuit unless current is flowing in that circuit. So if there’s a measurable amount of voltage drop at an individual fuse, regardless of how small, that indicates that current is flowing in the circuit that the tested fuse is protecting. If no voltage drop is observed at a particular fuse, it’s safe to assume that no current is flowing in the protected circuit at that time.

If a voltage drop is detected across an individual fuse, what does the meter reading mean? How does the meter’s voltage reading translate into a milliamp key-off battery drain current? This is where CTI’s George Lesniak’s charts that I mentioned last month come in. George has done the hard work for us, providing voltage-to-milliamp conversions for all of the major fuse types and amperage ratings. Measure the voltage drop at the fuse, find the fuse type and amperage on the chart, then follow the grid to find the equivalent milliamp reading.

The beauty of this test method is that everything in the vehicle remains undisturbed. There’s no need to disconnect battery cables or pull fuses to conduct a test and find a drain. This is of particular importance on late-model vehicles, which may contain dozens of electronic modules that go through a normal power-down sequence each time the ignition is turned off. Removing a cable or fuse during a test may disrupt this power-down sequence, forcing you to wait as much as a half-hour or more for any of the affected modules to safely go back to sleep. Only then can you begin to look for the source of any abnormal battery drains.

Since the column was published, I’ve received many requests via e-mail for copies of George’s charts. Most of the notes I’ve received simply asked for a copy of the charts, and I’m happy to oblige. Several readers also shared their key-off battery drain troubleshooting techniques with me, which has been especially gratifying. A couple of readers questioned whether it might be easier to simply remove a fuse, then plug an amp meter directly into the circuit. If any current were flowing in the circuit, no matter how small, it would show up on the amp meter display.

This method will certainly work, and test equipment is available for this purpose. The Fuse Buddy tools from Electronic Specialties (www.esitest.com) are a good example. These tools take the place of a fuse, allowing a direct amperage reading, which could be especially helpful once the offending circuit had been identified via voltage drop testing. The Fuse Buddy’s semipermanent connection at the fuse panel would allow you to conduct further tests on the components in the circuit, which could be very helpful in an intermittent battery drain situation.

I mentioned a couple of classic key-off battery drain sources last time, and of course there are others. A poor-quality or improperly installed aftermarket alarm system can drain a fully charged battery in a few days. The alarm may have more than one connection to the vehicle’s electrical system, and the installer may have tapped into the nearest available power source. Besides causing key-off battery drain problems, this can wreak havoc on engine management systems and other delicate electronic systems.

Aftermarket audio systems, especially the kind with huge power amps and pinky-sized wires running directly from the amps to the battery, are another potential source of key-off battery drain problems. The system’s control head is supposed to switch the power amps into standby mode when the ignition is off. But even in standby mode, they may draw a few milliamps. And if the head unit has not been properly installed or set up, the power amps may never power down, even when the ignition is off. In this case, the amps may draw several hundred milliamps and the battery will become discharged in short order.

A growing number of new vehicles are available with proximity keys, which function in a manner similar to an instant-on television and its remote control. A radio receiver in the vehicle listens for a signal from the proximity key. When a signal is received, the system must determine if the key is a match with the vehicle. The vehicle doesn’t know if the owner is going to be gone for an hour or a week, so it’s constantly listening and never goes completely to sleep. Every proximity key that passes within range of the receiver causes the system to consume more battery power for a few minutes as it attempts to identify the proximity key. So if the vehicle were parked in a busy parking structure, key-off battery drain could be accelerated.

The problem can be further aggravated if the vehicle is equipped with a relatively small battery. The 12-volt battery on the Toyota Prius is about half the size and capacity of the batteries found in conventional vehicles. Many Priuses are equipped with proximity keys. So it would be a good idea to inform your Prius customers that if they intend to leave their vehicles undriven for more than a few days, they should turn off the proximity key feature. The owner’s manual will provide instructions.

Once again, my thanks to George Lesniak of the Carquest Technical Institute (www.carquest.com/proCTI.html) for introducing me to a new parasitic battery load testing method. If you find that it’s made your life easier, be sure to pass it on to your fellow techs.